Conventional steel cylinders for use in plastic or rubber extruders and injection molding machinery comprise a series of relatively short tube segments made of tool steel assembled inside a larger tube known as a backing tube or backing material. Short tube steel segments are used because of heat-treating problems associated with longer thin-walled tubing. Typically, thin-walled tool steel tubes warp during the heat-treating process and crack when inserted into the straight bore of the backing tube for shrink fitting purposes. Manufacturers currently overcome this problem by keeping the length of the tool steel segments short.
Segmented tool steel liners have several inherent problems. While manufacturers claim that the segmented liners appear to be essentially seamless as a result of a honing process, the cracks between the segments are still there, even if they are initially microscopically small. During the operating life of the cylinder, the constant mechanical flexing caused by thermal and mechanical forces may cause the segments to separate slightly. When such conventional cylinders are used for plastic extrusion, colored plastic residue may get trapped in the cracks and contaminate a new colored plastic that is being processed.
Furthermore, cracks between the segments open due to normal wear on the tool steel liner bore as a result of processing certain plastic resins, especially highly abrasive plastics. Corrosiveness of the resin material being processed further deteriorates cylinder performance by attacking the unprotected backing material in the areas of the cracks.
While tool steel segments in conventional designs are typically held in place by means of an interference fit, typical manufacturing tolerances on the outside diameter of the tool steel segment and the corresponding inside diameter of the backing tube can result in variations in the interference fit. Thus, while one of the segments may held in place by be a true and severe shrink fit, another may be merely a line-on-line fit that generates very little or no real holding power. The short length of such a tool steel tube segment would provide no appreciable anti-rotational resistance.
The present invention has been developed in view of the foregoing, and to address other deficiencies of the prior art.